Regular Talk: A Sequent Calculus for Intersection and Union Logic

Abstract

We present a logical formalism for the
intersection and union type assignment system IUT [1]. A
first attempt to this end is the
intersection and union logic IUL, a logic whose main structures are binary
trees called kits [3,4]. The rules of IUL are in natural
deduction style and are categorized as global or
local according to whether they affect all leaves of the
kits involved or not. While rules for the intersection and union are meant to
be local, the complex notation of kits conceals a certain kind of globality
inherent in the union elimination rule. This becomes explicit if we abandon
kits and resort to the linear structures employed in [2]. These structures are
multisets of judgements (atoms) called
molecules. A logic for IUT using molecules, but still in
natural deduction style, incorporates a union elimination
rule

which has both a global and a local behaviour. Here globality and locality
refer to the number of atoms modified by the rule in the premise molecules.

In the present work we describe IUL with molecules
in sequent calculus style. The main advantage of this formulation is that
globality and locality of the union elimination rule are separated in the
cut rule
and left union rule, respectively.